The invention relates to a drying cylinder which is adapted to be heated with steam, particularly for paper making machines.
The invention can preferably be applied to so-called crepe cylinders, whose outside diameters may, in the extreme case, amount to more than 6 meters and which are intended for paper making machines which work at high speeds (of the order of 2,000 meters per minute). Crepe cylinders of this kind are heated with steam which is at relatively high pressure, because the high working speed gives rise to very high heat flow density in the cylinder jacket. The invention can, however, also be applied to glazing cylinders, although these are operated at lower working speeds and are heated with steam which is at lower pressure.
Prior art devices are disclosed in the following publications:
1. J. M. Voith GmbH Publication p 2253. PA1 2. German Auslegeschrift No. 1,160,712 equivalent to U.S. Pat. No. 3,099,543. PA1 3. Wochenblatt fur Papierfabrikation 1977, pages 447-458.
The essential parts of a drying cylinder include a hollow, tubular cylinder jacket, two cylinder covers covering over the ends of the jacket, and a hollow shaft entering the cylinder through at least one cover. To secure the jacket to the cylinder covers, the jacket is provided at each end with a flange of approximately rectangular cross-section. In above-noted Publication No. 1, the axial length of the flange cross-section is greater than its radial thickness. In addition, each cover has a connection flange whose cross-section is L-shaped. The L embraces the jacket flange in such a manner that centering is provided over a relatively great axial length between the jacket and the cover. The covers are curved in shape in the direction of the interior of the cylinder and have relatively thin walls.
The covers of the drying cylinders described in Publications Nos. 2 and 3 have only a very short centering surface at their connection flanges. The flanges of the cylinder jacket are accordingly also fairly short in the axial direction. In Publication 2 slightly curved cylinder covers are shown, while in Publication 3 the cylinder covers have a conical shape.
FIG. 15 of Publication 2 shows how a drying cylinder is deformed in operation. The cylinder jacket first expands as it heats up, and then it shrinks again as heat is extracted from it by the paper to be dried. On the other hand, as will be further explained below, the cylinder covers behave quite differently. Since the cover cannot follow the deformations of the cylinder jacket, relatively great stresses occur in the end regions of the cylinder jacket, near its flanges. Designers endeavor to ensure that these stresses are at most equal to the greatest stress occurring in the axial middle of the cylinder jacket. For achieving this aim, numerous different steps have previously been taken:
A. In FIG. 1 of Publication 2, the two cylinder covers are joined together by a plurality of tie rods. This is an attempt to ensure that the only slightly curved covers will bend outward under the steam pressure to a smaller extent than previously occurred (see FIG. 13 in comparison with FIG. 15). This arrangement, however, is heavy and expensive.
B. An attempt to achieve a similar effect is made with the constructon shown in FIG. 5 of Publication 2. Here the diameter of the hollow shaft is increased and the covers are curved outward. Here again the total weight is increased. In addition, the installed length in the region of the covers is greater than previously.
C. In Section 3 (Page 454) of Publication 3, it is stated that with conical covers, a reduction of the inside diameter of the cover (and hence of the diameter of the flange on the hollow shaft) and a simultaneous reduction of the wall thickness of the covers are advantageous. However, conical covers must in any case have a relatively great wall thickness in order to be able to withstand the steam pressure. Publication 3 then also points out the influence of the ratio between the length of the cylinder jacket and the width of the paper.
The constructions described at A, B and C above all have the additional disadvantage that because of the very short centering surfaces (in the axial direction), there is a danger that the covers will not follow the deformations of the cylinder jacket to the desired extent, so that the tightness of the flange connection will leave much to be desired. This danger becomes even greater if a coacting roller is pressed against the drying cylinder. An oscillating load is thereby superimposed on the previously mentioned stresses, and this may cause leakage in the flange connections.
D. For the drying cylinder according to Publication 1, a different method has been adopted from that employed in Publications 2 and 3, namely the previously mentioned shape of the flange connection (with a long centering surface), for ensuring that the stresses at the ends of the jacket will not exceed a permissable value. Complete tightness of the drying cylinder can also be ensured by the relatively stiff flange connection. Another advantage of this construction is the sharply curved shape of the covers. As a result, the covers can have only a slight wall thickness, while nevertheless being only slightly deformed under the steam pressure.
With the construction known from Publication 1, however, difficulties may arise if an extremely high working speed is required and accordingly there is a still further increased heat flow density. In such cases, it has been found that additional steps are necessary to reduce the stresses in the end regions of the cylinder jacket. This is particularly the case if superheated steam is used and if the covers are to be provided with thermal insulation on their outward sides in order to save energy. The covers may then assume superheating temperatures and expand accordingly. This expansion is transmitted by the covers to the cylinder jacket, on which the above-mentioned stresses are thus imposed. For these reasons, it has not previously been permissible to provide thermal insulation on the covers when superheated steam was used.